The present invention relates to a traveling-wave tube, and more particularly, to a traveling-wave tube employing a coupled-cavity type slow-wave circuit.
A coupled-cavity type traveling-wave tube is characterized by a higher withstand power than a helix type traveling-wave tube, and it is widely used as high power sources in microwave and millimeter-wave bands such as output power tubes in ground stations for satellite communication. In cases where it is used for communication lines of large capacity and high quality as is the case with satellite communications, mostly it is used in a region having a good linearity, that is, in a small signal operation range where the output power level of the tube is low, so that small signal gain versus frequency characteristics serve as important factors for determining whether the tube is good or bad. However, the coupled-cavity type traveling-wave tube has a disadvantage that the operating band width of its small signal gain versus frequency characteristics is extremely narrow as compared with that of a traveling-wave tube employing a helical structure slow-wave circuit. Therefore, as means for extending this band width the following techniques have been proposed. That is, according to a disclosure in Japanese Patent Publication No. 42-3973 it has been proposed that a protrusion for concentrating a magnetic field is formed in the proximity of a coupling iris between adjacent cavities to increase the pass-band width of a slow-wave circuit and thereby the operating band width of a tube can be broadened. However, if the pass-band width of a slow-wave circuit is increased in this way, then a coupling impedance of this slow-wave circuit is reduced, so that there was a disadvantage that a gain and an efficiency of a tube were lowered. In addition, according to a disclosure in Japanese Patent Publication No. 44-16090, a method is known in which approximately at the center of a slow-wave circuit is provided a distributed attenuator region as spaced from a sever section at a predetermined interval, and thereby oscillation at a band edge is prevented and also an operating band width of a tube is broadened. However, this method has a disadvantage that because of increase of attenuation in the output slow-wave circuit, the gain is largely lowered.
Heretofore, when a a coupled-cavity structure slow-wave circuit is employed in a travelling-wave tube, it has been the common practice to sever the slow-wave circuit into two sections consisting of one preliminary or fore slow-wave circuit (input slow-wave circuit) and an output slow-wave circuit for the purpose of suppressing oscillation caused by a reflected wave from its output end as shown in FIG. 2 on page 1832 of an article by R. J. Collier, G. D. Helm, J. P. Laico and K. M. String entitled "The Ground Station High-Power Traveling-Wave Tube" (The Bell System Technical Journal, July 1963), or to sever the slow-wave circuit into three sections consisting of two preliminary slow-wave circuits (an input slow-wave circuit and an intermediate slow-wave circuit) and an output slow-wave circuit for the same purpose in the case where the traveling-wave tube has a higher gain as shown in Japanese Patent Publication No. 44-16090. In these prior art tubes, each of the severed slow-wave circuits consists of coupler cavities positioned at its opposite ends and a number of main cavities intervening between these coupler cavities, and the coupling cavity is normally formed smaller in diameter or in height than the main cavity in order to provide characteristic impedance matching with a waveguide to be connected to this coupler cavity. Cavity diameters, heights and sizes of coupling irises between adjacent cavities of the respective main cavities, interaction gap dimensions of the respective cavities, and distances between centers of interaction gaps in the adjacent cavities, are equal throughout the respective slow-wave circuits, and therefore, TM.sub.01 cavity mode pass-band widths and phase velocities of traveling waves are equal in the respective slow-wave circuits. The phase velocity is a function of the frequency, and in the prior art, a frequency for giving a phase velocity equal to a D.C. beam velocity, that is, a synchronizing frequency is often selected at an intermediate point between a lower end frequency of a desired operating band and a TM.sub.01 cavity mode lower cut-off frequency of a slow-wave circuit in view of an efficiency of a tube. As a result of these provisions, maximum gain is obtained at the synchronizing frequency outside of the operating band, and within the operating band the gain is lowred as the frequency is raised although there exist some gain ripples caused by internal reflection waves in the slow-wave circuit, so that even if a TM.sub.01 cavity mode pass-band width of a slow-wave circuit should be extended by the above described method, the operating band width would be limited to 30.about.40% of the TM.sub.01 cavity mode pass-band width.